For every modern firearm, whether manual or automatic, six steps of operation must be accomplished after firing. Those steps include unlocking the action, extraction of the spent cartridge casing from the firing chamber, ejection of the spent casing, cocking the hammer for the next round, feeding a new round into the firing chamber and locking the action back into battery. In an autoloading firearm, each of these steps is performed by the firearm itself. A semiautomatic firearm performs these steps for each pull of the trigger.
There are three main types of systems for semiautomatic firearm operation: recoil, blow back and gas. In the first of these, the firearm's recoil or kick thrusts the barrel and breech-bolt rearward to operate the action. Conversely, in blow back operation, recoil is used to force the breechblock back without moving the barrel. Each of these systems has been adapted to a wide range of hand-fired and shoulder-fired guns.
The third system for semiautomatic firearm operations has become the most prevalent. Gas-operated firearms and gas-operated shotguns in particular have gained wide acceptance and popularity with firearms enthusiasts. Gas-operated mechanisms, as opposed to blow back or recoil-operated mechanisms, provide a number of advantages. For example, the speed of fire for semiautomatic weapons may be much higher. Also, the heavy operating parts needed by recoil and blow back systems can be avoided, particularly with high-powered cartridges. Thus, overall gun weight is reduced. Moreover, gas operation can be more precisely controlled to generate only the amount of force necessary to cycle the firearm action. Gas operation also provides the additional benefit of spreading the force of recoil out over a longer time frame therefore making the recoil less noticeable and therefore less objectionable to the shooter. Controlling the recoil force in this manner also tends to make the weapon more accurate.
An early gas-operated mechanism for autoloading firearms is disclosed in U.S. Pat. No. 3,200,710 to Kelly, et al. ('710), assigned to the same assignee as this application. The contents of that patent are incorporated herein by reference in its entirety.
In one type of gas operated firearm, a gas port or small hole in the barrel allows propellant powder gas to enter a gas cylinder and to exert pressure on some kind of piston. The piston engages a connecting rod at a forward end with the rear end connected to the firearm action. As the piston is driven rearward by the rapidly expanding propellant gas pressure, the connecting rod works the gun action to perform the steps outlined above. A return recoil spring typically located in the gun stock provides the force necessary to return the action to battery after the spent shell casing has been extracted and ejected. The piston must withstand the repeated subjection to the violent thrusting forces generated by the propellant gas and also the corrosive nature of that gas.
Many of today's gas operated shotguns feature a gas chamber created around the gun magazine tube which is located adjacent to and directly beneath the gun barrel. The gas chamber resides between the outside surface of the magazine tube and the inside surface of a cylindrically-shaped member, commonly called a gas cylinder, connected to the barrel. The piston in this design is a circular member which encircles the magazine tube inside the gas chamber. During the piston's travel through the gas chamber, a gas seal must be provided between the piston and the magazine tube and the piston and the cylindrical member to take full advantage of the powder gas force.
The piston and piston seal described in the '710 patent operate as described above and provided a substantial performance improvement over other designs of the time. However, it has been found that this design may experience stress crack failures. This failure in some instances may render the firearm inoperative, leading those consumers to become dissatisfied with the perceived quality of the firearm. The failures are believed to stem from twisting and/or torquing of the piston and piston seal as they travel along the magazine tube. The relatively small length to diameter ratio of these components may contribute to their susceptibility to twist induced stress cracking. Also, the inside diameter of each component is in sliding contact with the magazine tube that over time may become fouled with deposits and residue from the powder gas. These deposits may cause the piston and piston seal to bind during their travel along the tube.
Thus, there is a need then for a new gas chamber sealing device which will provide the required gas seal, improve service life and address the additional problems described above.